Locked out of my hardrive - I think my Roommate hacked me

I've been having some problems with my network and OS recently that I think go back quite a while. Though I've had my computer for a little over a year and a half, I'm not very knowledgeable when it comes to network and OS problems, so please bare with me. I'm running a 13" Macbook with 10.5.8 OS X.

Sometimes my roommate's computer pops up under the 'shared' part of Finder. It only happens on occassion though I know he's often on the internet, and I've set my system to share nothing in System Preferences. This has been going on for a few months - ever since, one day, he spent an hour on the phone with our ISP, then needed to 'borrow my computer quickly to check his email', and then reset the router. We have paper-thin walls and I can hear him talking on the phone, to friends he has over, and to my other roommate - and he ALWAYS knows what I've been doing. One night he and a friend were actually talking about what I was doing physically at my desk, and now he mutters when I cover up my webcam.

This bothered me immensely, and I realized that I've done practically nothing to secure my computer. I set up my firewall and I want to run FireVault but I can't clear enough space on my hardrive. While I was installing Anti-Virus and network-scanning programs on Saturday, my hardrive was suddenly locked to me. Now it tells me that I do not have sufficient priveldges to access it.

I'm going to kill someone. I've been told that I need to reformat my hardrive in order to access it again. If there's been a proxy set up to eavesdrop on my screen, or some ARP-table foulplay, how do I detect it, how do I fix it, and how do I make sure it doesn't happen again?

what eidorian said.. the only way you will be able to get back into your account, i suspect is by getting an external, installing OSX on it, booting into said HD, then you should be able to repair permissions, and apply some firewall etc etc.

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